The nephridium (plural nephridia) is an vertebrate kidney. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other types.
A metanephridium (meta = "after") is a type of Bojanus organ.)
A metanephridium typically consists of a ciliated funnel opening into the body cavity or nephridiopore. The primary urine produced by filtration of blood (or a similar functioning fluid) is modified into secondary urine through selective reabsorption by the cells lining the metanephridium.
The saccate metanephridia are excretory glands which function similarly to the metanephridia. They are found in the arthropods. The saccate metanephridia filter the fluid of the hemocoel, as opposed to the metanephridia which filter coelomic fluid. In a saccate metanephridium, there is a ciliated funnel covered with a membrane that helps to filter the hemocoel of heavy particles (such as proteins and carbohydrates) before the fluid even enters the funnel. Inside the funnel, the fluid is further processed through selective reabsorption, and eventually excreted from the nephridiopore. In crustacea, the saccate metanephridia are associated with the antennae and form the antennal gland. In freshwater crustacea, the saccate metanephridia are especially large because they are involved in osmoregulation. They have to remove large amounts of water from the tissues, as the crustacean's tissues are hypertonic to the surrounding water.
A protonephridium (proto = "first") is a network of dead-end osmoregulation).